Modern electronic equipment has become so sophisticated in recent years that even the simplest products now include microprocessors and memories. Being price competitive has dictated the use of inexpensive random access memories (RAM) for storing large amounts of data. Although RAM devices do not consume a great deal of power, data are lost when primary power is completely removed--as is the situation during commercial power outage, battery replacement, and inadvertent removal of a power cord. While this may not present a serious problem in some products, it is not desirable to require customers to reprogram their feature-rich telephone sets whenever they are unplugged and moved to a new location. Likewise, it is undesirable to update digital clocks and radios in similar circumstances.
One common approach to dealing with this problem is to provide a backup battery that keeps memory intact whenever primary power is lost. Such batteries are generally customer purchased and installed since they are seldom included with the product. Unfortunately, replaceable batteries usually require a battery test circuit which adds to product cost. Replaceable batteries further require storage compartments that increase the size and cost of such products, and must be positioned to facilitate customer access. Battery compartment doors are frequently lost or broken, which not only detracts from product appearance, but also eliminates needed support for the battery. Accordingly, it is desirable to avoid the use of customer-installable batteries and associated battery compartments.
Another approach to dealing with this problem is to permanently install a battery in the product so that the need for customer access is eliminated. Unfortunately, battery shelf life can be a problem, particularly if the battery is being drained before the product is even sold. Indeed, the product may sit on a shelf in a retail store or in a warehouse for more than a year before it is purchased by the customer. During this time, exposure to elevated temperatures is possible, particularly during warehouse storage; and it is at these elevated temperatures that current requirements increase dramatically for both JFET and MOS devices thereby draining permanently-installed batteries at an unacceptably high rate.
Yet another approach to the problem of keeping volatile memory alive when primary power is absent in the use of batteries that can be re-charged during the normal operation of the product. Rechargeable (Nickel-Cadmium) batteries are relatively expensive, however, and in telephone-line powered equipment receive only a small amount of charging current when the telephone handset is off-hook, and practically none when it is on-hook. Notwithstanding the fact that such batteries can be re-charged, they have reliability problems and eventually wear out. A variation on this approach is the use of a large capacitor which is charged whenever primary power is present. Unfortunately, such capacitors do not store sufficient energy to provide reliable backup power for extended time periods.
Special controllers exist (e.g., DS1234 Conditional Nonvolatile Controller Chip from Dallas Semiconductor) that protect volatile memory from power loss via software-controlled switches. Delivery of an "enable" code to the controller causes the switch to operate, and delivery of a "disable" code causes it to release. It is known to use such switches to enable/disable a battery during shipment or storage. Unfortunately, such switches can be inadvertently disabled, enable codes are relatively complex, and these codes cannot be stored in volatile memory if battery power is to be conserved during shipment or storage.